Hello!
I have a homehub 5 that I have bought already installed with OpenWRT 17.01.6 r3979-2252731af4
I see that 19.07 is released for my HW, so I was considering an upgrade.
Can I just backup the settings from LUCI, upgrade the new bin file from LUCI and reload the settings and that's it ? or shall I expect troubles (like redo my whole configurations manually?)
If it does make a difference, I have similar question with a Buffalo WCR-1166DS that I would upgrade from 18.06.2 to 19.07 too.
Better go for a clean installation and restore the settings manually, especially for the HH5 which is running 17.01
The other one you might get lucky and be able to restore the configs after the clean installation. But if you experience any hickups, reset to defaults and do it manually.
Sounds like a nightmare 
Maybe I can update from 17 to 18 first? 
This is not the case. The config file will not be adapted to the new version when you upgrade from 17 to 18 and then to 19. So all the deprecated options and settings of 17 will move to 19 one way or the other.
You can always try to upgrade with settings and see if this works for you, but you have been warned in case it backfires.
ok tested yesterday, perfect upgrade, no settings lost 
Only a few packages to reinstall (the ones I previously installed myself).
I just used this script to list the packages before upgrading.
fwiw, download and read the Installation Guide for HH5A
https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=266
https://www.dropbox.com/sh/c8cqmpc6cacs5n8/AAA2f8htk1uMitBckDW8Jq88a?dl=0
Packet steering isn't configured/working correctly in 18.06 and 19.07 by default on xrx200 devices like HH5A.
This packet steering issue : do you refer to this wan to lan throughput issue ? If yes, it's no trouble for me. I use this homehub as dumb AP + ethernet switch. I don't use it as router.
I may use the wan port as a 5th lan port someday, if yes, I will use it for something that don't need speed.